This invention relates to an acoustic focussing device, particularly for the focussing of ultrasonic and shock waves for the no-contact crushing of a concrement disposed in the body of a living being.
For the focussing of flat or slightly curved shock wave fronts, as they are generated in the case of lithotrity instruments which operate, for example, according to the electromagnetic or piezoelectric converter principle, an acoustic lens system is required. In body, the focussed shock wave is aligned with the stone to be treated. In this case, depending on the position of the stone, different penetration depths of the shock wave are required.
The requirement of the variable penetration depth can be met by systems with a fixed focal length and with an additional variable forward-flow path (such as a bellows-shaped water cushion) or by a system with a variable focal length.
Other requirements with respect to a therapy unit for lithotrity are, for example, the overall size, the weight as well as technical expenditures that should be as low as possible in the case of the peripheral equipment (such as a position-independent, sensitive pressure/volume control).
In the German Patent Document DE 85 23 024 U1, an ultrasonic generator is indicated which has a deformable boundary surface between the coupling surface to the patient's body and a piezoelectric converter, the curvature of this boundary surface being changeable by the change of the pressure in the adjacent liquid. As an alternative, the focus displacement may also be achieved by the shifting of an additional solid-state lens.
In the German Patent Document DE 37 39 393 A1, a lithotritor is described with an adjustable focussing in which the wall of a liquid immersion objective is connected with a part of an adjusting device. By moving the adjusting device in the shock wave propagation direction, the curvature of the wall will change.
In the German Patent Document DE 33 28 051 A1, a device is described for the no-contact crushing of concrements in which the change of the focal point is achieved by the shifting of one or several acoustic lenses.
From the German Patent Document DE 36 05 277 A1, a shock wave therapy device is known in which a lens is surrounded by the coupling medium, in which case the liquid areas in front of and behind the lens are connected with one another.
It is an object of the present invention to provide a focussing device of a very small overall size whose focal length (focal intercept) can be varied within a wide range and, in addition, reduces the technical expenditures in the case of therapy instruments. In this case and in the following, the focal length F or the focal intercept of a lens system is the distance between the focus and the closest point of the--viewed from the direction of the shock wave source--last refractive surface of the lens system.
The object is achieved according to preferred embodiments of the invention by providing a system comprising:
a housing,
a plurality of boundary surfaces in the housing which define gap spaces arranged behind one another in a sound propagating direction of sound waves to be focussed,
a first fluid disposed in one of said gap spaces,
a second fluid disposed in another of said gap spaces which is separated from the first gap space by one of the boundary surfaces, said one boundary surface being flexibly deformable,
said first and second fluids exhibiting different sound transmission velocity characteristics,
and a boundary shifting device for shifting at least one of the boundary surfaces in parallel to a sound propogating direction of sound waves to be focussed such that at least one of said fluids is displaced between two of the gap spaces with consequent deformation of the one flexibly deformable boundary surface to change the radius of curvature thereof and thus change the focus of said focussing device.
According to the invention, several boundary surfaces are arranged behind one another in the propagation direction of the sound waves, in which case adjacent gaps contain liquids of different sound velocities. In this case, these boundary surfaces may consist of material s which are nondeformable; that is, their form is particularly not affected by pressure differences between liquids bordering on both sides of the boundary surface. However, at least one of the boundary surfaces is made of a deformable material; that is, a deforming of this boundary surface as a result of pressure differences between the bordering liquids is possible. At least one of the boundary surfaces may be displaced in parallel to the propagation direction of the sound waves and may then be locked in its position. At least one gap is connected with a non-adjacent gap.
By means of the shifting of the movable boundary surfaces, the liquid is displaced between the connected gaps, and as a result the radius of curvature of at least one of the deformable boundary surfaces is changed.
A change of the refractive characteristics of the whole focussing device, particularly the refractive power and the local length F, therefore finally results in two effects:
1. Change of position of one or several shifted boundary surfaces; PA0 2. Change of refractive power of one or several deformable boundary surfaces because of their deformation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.